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ECET 211 Electric Machines & Controls

Relays

Text Book: Electric Motors and Control Systems, by Frank D. Petruzella, published by McGraw Hill, 2015.

Paul I-Hai Lin, Professor

Electrical and Computer Engineering TechnologyP.E. States of Indiana & California

Dept. of Computer, Electrical and Information Technology

Purdue University Fort Wayne Campus

Prof. Paul Lin

Lecture 7 Relays

Part 1. Electromechanical Control Relays

• Relay Operations

• Relay Applications

• Relay Styles and Specifications

Part 2. Solid State Relays

• Operation

• Specifications

• Switching Methods

Part 3. Timing Relays

• Motor-Driven Timers

• Dashpot Timers

• Solid-State Timing Relays

• Timing Functions Prof. Paul Lin 2

Part 4. Latching Relays

• Mechanical Latching Relays

• Magnetic Latching Relays

• Latching Relay Application

• Alternating Relays

Part 5. Relay Control Logic

• Control Circuit Inputs and Outputs

• AND Logic Function

• OR Logic Function

• Combinational Logic Functions

• Not Logic Function

• NAND Logic Function

• NOR Logic Function

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Part 1. Electromechanical Control RelaysRelay Operations

Electromagnetic Relay – a switch that is operated by an electromagnet

Two parts of Electromechanical Control Relay (Figure 7-1):

• Coil input (control circuit)

• Contacts output (Load circuit)

Used to control small load of 15A or less

Applications

• Control coils in motor contactors and starters

• Switching solenoids, pilot lights, audible alarm, and small motor (1/8 hp or less)

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Part 1. Electromechanical Control RelaysRelay Operations

Figure 7-2 Relay coil and contacts

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Typical Relay Applications: One relay controls multiple pilot lights

Figure 7-3 Relay coil and contacts – CR1, CR2

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Part 1. Electromechanical Control RelaysRelay Applications

Current or signal amplification (small signal current => control large load current)

Control high-voltage load: 12 V => 480V load

Figure 7-4 Relay used to control a high-voltage

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Control high-current load

Figure 7-5 Using a relay to control a high-current load circuit with a low-current control circuit

• Relay: coil and contact

• Solenoid (2 A, 120V)

• Electronic parts: Diode, Transistor, Resistor (limit current)

Part 1. Electromechanical Control Relays

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Theory of Operation – Figure 7-5 Control High Current Load

Relay coil - on/off control by a power Transistor switch

Relay switching on/off induced high Spike voltage e = - L ∆i/∆t

Transistor: • Three terminal device: Collector,

Emitter, Base

• Base input current 2 mA, Collector output current 20mA: a gain of 10

Diode (fast on/off switch)–protecting transistor: fly wheeling diode turned-on, when VA – VK ≥ 0.7 V

Solenoid value current 2 A

• Total Current Gain = 2A/2mA = 1000

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Part 1. Electromechanical Control RelaysRelay Styles and Specifications

Figure 7-6 Plug-in style ice cube relay

Figure 7-7 Relay manual push-to-test button

Figure 7-8 Typical DIN-rail with relays mounted

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Part 1. Electromechanical Control RelaysRelay Styles and Specifications

Figure 7-9 Common relay contact switching arrangement

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Part 2. Solid State RelaysOperation

Solid-State Relay (SSR)

• An electronic switch contains no moving parts

• Does not have coils and contacts

• Use semiconductor switching devices: Bipolar transistor, MOSFET, SCR, Triac

Figure 7-10 Typical solid-state relay

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Part 2. Solid State Relays

Operation

Figure 7-11 Optically coupled SSR used for AC load

Figure 7-12 Optically coupled SSR used for DC loads

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Part 2. Solid State RelaysSpecifications

Input voltage range: 5V DC to 24V DC, 32 V DC

Output voltage range: 5V DC up to 480V AC

Current rating: < 10A, up to 40 A, 50A (heat sink required)

Figure 7-13 Multiple-pole SSR connections

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Part 2. Solid State RelaysSpecifications

Figure 7-13 Three-wire control utilizing a SSR and an SCR

SCR (Silicon Control Rectifier)

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Part 2. Solid State RelaysSwitching Methods

Zero-switching relay (Zero crossing)

Peak-switching relay

Instant-on relay

Thermal dissipation

Leakage current

Cost

Figure 7-15 Zero Crossing SSR

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Part 2. Solid State RelaysSwitching Methods

Figure 7-16 SSR heat sink

Loads more than 5A require a heat sink for reliable operation

Figure 7-17 Electromechanical vs. SSR construction

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Part 3. Timing RelaysTiming Relays – An Introduction

A time actuated control relay in which a fixed or adjustable time occurs after a change in the control signal before switching action occur.

Enable a multitude of operation in a control circuit to be automatically started or stopped at different time intervals

Figure 7-18 Timing relays

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Part 3. Timing RelaysMotor-Driven Timers

Figure 7-19 Synchronous clock timer

Dashpot Timer

Figure 7-20 Dashpot (Pneumatic) Timers

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Part 3. Timing RelaysSolid-State Timing Relays

Figure 7-21 Solid-state timing relay connections

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Part 3. Timing RelaysSolid-State Timing Relays

Timing Functions

On-Delay Timer

• Figure 7-22 On-delay timer contacts

Off-Delay Timer

One-Shot Timer

Recycle Timer

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Part 3. Timing RelaysTiming Functions – On delay timer

Figure 7-23 On-delay timer circuit

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Part 3. Timing RelaysTiming Functions - Off-Delay Timer

Figure 7-24 Off-delay timer

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Part 3. Timing RelaysTiming Functions – Off Delay Timer

Figure 7-25 Off-delay timer automatic pumping circuit

Water level rises to point A => Level sensor contact on => Energize Off-delay timer => Turns on the pump to initiate the pumping action

Water level decreases => Level sensor contact open => Timing begin => The pump Continue to run and empty the tank for the length of the delay time

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Part 3. Timing RelaysTiming Functions - One-Shot Timer

Figure 7-26 One-shot timer

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Part 3. Timing RelaysTiming Functions - Recycle Timer

Figure 7-27 Recycle timers

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Part 3. Timing RelaysMultifunction Timer – a timer that perform more than one timing function

Figure 7-28 Multifunction digital timer

H5CX-N, 0.001 s to 9999 h,http://industrial.omron.us/en/products/catalogue/control_components/timers/digital_timers/default.html

PLC Timers

Most common types of PLC timer instructions

• On-delay timer (TON)

• Off-delay timer (TOF)

• Retentive Timer On (RTO)

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Part 3. Timing RelaysFigure 7-29 PLC programmed on-delay timer

Hardware wire

• Pressure Sw => I3 (input)

• Pilot Light => Q1 (Output)

Enter Ladder diagram using keypad and LCD display

PLC Program Execution

• Pressure SW on => energize Timing coil T1 => Initiating the time-delay period

• After 5 sec have passes, T1 (NO contact) closed to energize relay coil Q1 and turn on the pilot light

• Open Pressure SW, resets the time value to zero

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Part 4. Latching RelaysLatching Relays – use a mechanical latch or permanent magnet to hold the contacts in their last energized position.

Mechanical Latching Relays

Figure 7-30 Two coil mechanical latching relay

Figure 7-31 Operation of a two-coil latching relay circuit

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Part 4. Latching RelaysMagnetic Latching Relays

Polarity sensitive

Relay is latched when the applied voltage with a predetermined polarity.

• A permanent magnet is used to hold the relay in its latched position.

A voltage polarity is reverses =>

unlatch the relay

Figure 7-32 Single-coil magnetic latching relay

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Part 4. Latching RelaysLatching Relay Application

For those applications that need to conserve the power, such as a battery-operated device

Figure 7-33 Battery-operated latching alarm circuit

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Part 4. Latching RelaysAlternating Relay (Impulse Relays)

It transfers the contacts with each pulse.

For special applications where the optimization of load usage is required by equalizing the run time of two loads.

Figure 7-34 Alternating or pulse relay

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Part 4. Latching RelaysAlternating Relay (Impulse Relays): Pumping Application

Two identical pumps are used for the same job.

• One for normal running

• One for backup

Running Methods

1) A standby unit is made available in case the first pump fail. However, a completely idle pump might deteriorate and provide no safety margin.

2) Both pumps get equal run time, alternately

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Part 4. Latching RelaysAlternating Relay (Impulse Relays): Pumping Application

Figure 7-35 Typical alternating relay circuit used with a duplex pumping system

• Use an Alternating Relay to ensure that both pumps get equal run time.

• In the off state, the Float SW is open, the Alternating relay is in the Load A position; both Loads (M1 and M2) are off.

• When the Float SW closes, it energizes the first load (M1) and PL1.

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Part 4. Latching RelaysAlternating Relay (Impulse Relays)

Figure 7-36 DPDT cross-wired contact version of a dual pumping application

Alternate Mode

• Normal Level

Lead and lag Flow SWs close simultaneously => Pumps A and B on (for heavy flow)

Only one pump is operating most of the time; yet the system has the capacity to handle twice the load

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Part 5. Relay Control Logic

Control Circuit Inputs and Outputs

Figure 7-37 Typical inputs and outputs of a control ladder diagram

• Input section

• Output section

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Boolean Algebra

Also see - Digital Logic (pages 257-258)

Boolean Logic

Logic AND

Logic OR

Logic NOT

Truth Table

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AND OperationM Coil ON = STOP (closed) AND START (ON)ORM Coil ON = STOP (closed) AND M Auxiliary Contact Closed

STOP START

M

V+ V-

CoilM Auxiliary

Contact

OR OperationM Coil ON = SATRT (ON) OR M Auxiliary Contact Closed

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Boolean Algebra – Logic AND

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V-

AND OperationM = A AND B

A B

M

V+

Coil

V-

AND OperationM = A AND B

A B

M

V+

Coil

A B M0 0 0 0 1 01 0 01 1 1

Truth TableInputs Output

Boolean EquationM = AB

Boolean Algebra – Logic OR

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Boolean Algebra – Logic NOT

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A

M

V+

Coil

V-

A

Standard Digital NOT Gate

M

A M0 1 1 0

Boolean EquationM = NOT A

Truth TableInputs Output

Boolean Equation

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A B C F

0 0 0 00 0 1 10 1 0 00 1 1 11 0 0 01 0 1 11 1 0 11 1 1 1

Truth TableInputs Output

Boolean EquationF = (AB) + CF = (A AND B) OR C

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Part 5. Relay Control Logic

AND Logic Function

Figure 7-38 AND logic function

OR Logic Function

Figure 7-39 OR logic function

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Part 5. Relay Control Logic

Combinational Logic Functions

Figure 7-40 AND/OR combinational logic

Both (On/Off switch AND Limit switch) must be ON AND (Sensor contact or PB be ON) => Contactor coil ON

Not Logic Function

Figure 7-41 Not logic function

Use to prevent accidental contact with live electrical connections.

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Part 5. Relay Control Logic

NAND Logic Function

Figure 7-42 NAND logic function

For dual-tank liquid filing operation

NAND = NOT (FS-1 OFF AND FS-2 OFF) => Pump starter coil (OFF) => Pump Motor shut down

NOR Logic Function

Figure 7-43 NOR logic function

NOR = NOT (Stop-1 OFF, Stop-2 OFF, Stop-3 OFF)

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Summary & Conclusion

Questions?Contact Prof. Lin through:

Email: lin@ipfw.edu

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